The present invention relates to porous composite particles and a process for producing the porous composite particles, and more particularly, to porous functional composite particles having not only a high catalytic by incorporating therein a large amount of a functional filling material having various catalytic property, but also a high adsorption ability to malodorous substances and harmful substances by uniformly distributing fine pores having an average pore diameter of especially not more than 5 nm in the surface and inside portions thereof, thereby exhibiting an excellent catalytic activity, and a process for producing such porous functional composite particles.
Hitherto, various studies have been made to solve problems of environmental pollution by chemical substances. In particular, in recent years, dioxins contained in an exhaust gas discharged upon incineration of combustible wastes cause significant social problems.
Also, with the recent tendency toward comfortableness, safety, hygiene and ecology, so-called amenity catalysts for living equipment or household appliances such as deodorization catalysts, water-treatment catalysts and combustion catalysts have been favorably used in ordinary amenity spaces. For this reason, it has been strongly required to provide catalysts for these purposes capable of exhibiting an excellent catalytic activity.
In order to remove malodorous substances or harmful substances in air, there have been used (1) a method of absorbing these substances into acid or alkali solutions; (2) a method of oxidizing these substances by using an oxidizing agent such as potassium permanganate and ozone, or by combusting at a high temperature; (3) a method of absorbing or adsorbing these substances into an adsorbent such as activated carbon and zeolite; or the like.
However, the method (1) cannot exhibit a sufficient removal effect, and rather tends to cause additional problems such as treatment or disposal of the resultant waste liquid containing the harmful substances or the like. In the method (2), the waste gas produced after the oxidation treatment must be post-treated to remove chlorine or ozone contained therein. Further, in the case where the high-temperature combustion is conducted, increased costs are required therefor. In the method (3), the absorbent used therein exhibits only a limited absorption capacity, thereby requiring complicated regeneration or replacement operations.
In order to solve these problems, there have been conventionally proposed various methods for improving the catalytic activity of catalysts used. For example, there are known a method of using a fluidized bed catalyst comprising a porous catalyst carrier having a surface area of 1 to 350 m2/g and a pore volume of 0.4 to 3.0 cc/g such as silica, alumina or zirconia, and a catalyst component such as iron, cobalt or nickel which is carried on a pore surface of the carrier in an amount equivalent to one atom layer (Japanese Patent Application Laid-Open (KOKAI) No. 58-109138(1983)); a method of using a carbon monoxide conversion catalyst formed by carrying a catalyst component containing chromium as a main component on porous heat-resistant zeolite (Japanese Patent Application Laid-Open (KOKAI) No. 61-8138(1986)); a method of using a mixture of activated carbon as an absorbent and calcium oxide, iron oxide, manganese oxide, aluminum oxide or the like as a promoter for decomposition of pollutants (Japanese Patent Application Laid-Open (KOKAI) No. 4-110035(1992)); or the like.
Also, there are known spherical composite particles composed of magnetic particles and carbon (Japanese Patent Application Laid-Open (KOKAI) No. 6-231931(1994)).
At present, it has been strongly required to provide catalysts capable of exhibiting an excellent catalytic activity. However, such catalysts fulfilling the above requirements have not been provided yet.
Specifically, in the method described in Japanese Patent Application Laid-Open (KOKAI) No. 58-109138(1983), the catalyst is carried on the pore surface of the carrier only in such a small amount corresponding to five atom layers at most, i.e., the amount of the catalyst itself carried thereon is insufficient, thereby failing to exhibit a sufficient removal effect.
In the method described in Japanese Patent Application Laid-Open (KOKAI) No. 61-8138(1986), the amount of the catalyst carried is also as small as 2 to 20 parts by weight based 100 parts by weight of the carrier, thereby failing to obtain a sufficient removal effect. Further, the porous carrier used therein is in the form of a honeycomb-like or hollow-cylindrical molded product and, therefore, can be used only in limited applications.
In addition, in the method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-110035(1992), since the individual particles cannot exhibit both of adsorption property and catalytic, it is difficult to effectively adsorb and decompose pollutants and harmful substances.
In the method described in Japanese Patent Application Laid-Open (KOKAI) No. 6-231931(1994), the spherical composite particles used therein are so designed as to achieve a dense structure composed of magnetic particles and carbon in order to keep the strength of the composite particles and, therefore, do not have a porous structure having pores therein.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by polymerizing phenols with aldehydes in an aqueous medium in the presence of a filling material to produce granulated composite particles containing a phenol resin as a binder; subjecting the reaction mixture to solid-liquid separation to separate the obtained granulated composite particles therefrom; and heat-treating the composite particles at a temperature of 400 to 800xc2x0 C. in an inert atmosphere to carbonize the phenol resin, the obtained porous composite particles have not only a high catalytic (ability as catalyst) but also a high adsorption ability, thereby exhibiting an excellent catalytic activity. The present invention has been attained based on the finding.
It is an object of the present invention to provide porous composite particles having not only a high catalytic but also a high adsorption property, thereby exhibiting an excellent catalytic activity.
It is another object of the present invention to provide a process for producing porous composite particles having not only a high catalytic but also a high adsorption property.
To accomplish the aims, in a first aspect of the present invention, there are provided porous composite particles comprising 80 to 98% by weight of a filling material and 2 to 20% by weight of carbon, and having an average pore diameter of not more than 5 nm when measured with respect to pores existing in both the surface and inside portions of the particles, an average particle size of 1 to 1,000 xcexcm and a specific surface area of 45 to 200 m2/g.
In a second aspect of the present invention, there is provided a process for producing porous composite particles which comprises the steps of polymerizing phenols with aldehydes in an aqueous medium in the presence of a filling material using a basic catalyst as an initiator to produce granulated composite particles containing a phenol resin as a binder; subjecting the reaction mixture to solid-liquid separation to separate said granulated composite particles therefrom; drying the obtained granulated composite particles to obtain dried composite particles containing a water-soluble phenol resin-forming component in an amount of 0.6 to 5 mg/liter when measured according to the test method for utensils and toys 3,1,5,3 defined in the Japanese Food Sanitation Law; and heat-treating the dried composite particles at a temperature of 400 to 800xc2x0 C. in an inert atmosphere to carbonize the phenol resin.
The present invention will be described in detail below.
First, the porous composite particles according to the present invention is described.
The porous composite particles of the present invention are particles comprising a filling material and carbon.
The filling material used in the present invention is composed of inorganic compound particles having various catalyst properties which are insoluble in water or undergo neither deterioration nor modification when exposed to water. Examples of the filling materials may include metal oxide particles such as iron-based oxide particles, titanium oxide particles, calcium oxide particles, silicon oxide particles, manganese oxide particles and aluminum oxide particles; various metal particles such as iron, nickel and magnesium; or the like. Specific examples of the iron-based oxide particles may include iron oxide particles such as hematite particles, magnetite particles and maghemite particles; particles formed by coating with or incorporating an element other than iron such as cobalt or the like onto or into the iron oxide particles; magnetoplumbite-type ferrite particles containing barium and/or strontium; spinel-type ferrite particles containing at least one metal selected from the group consisting of manganese, nickel, zinc, lithium and magnesium; or the like.
The amount of the filling material used is usually 80 to 98% by weight based on the weight of the porous composite particles. When the amount of the filling material used is less than 80% by weight, the amount of the filling material contained in the porous composite particles may become insufficient, thereby failing to obtain a sufficient catalytic. When the amount of the filling material used is more than 98% by weight, the obtained porous composite particles may be deteriorated in strength. In the consideration of both the catalytic and strength, the amount of the filling material used is preferably 80 to 95% by weight, more preferably 80 to 92% by weight based on the weight of the porous composite particles.
The amount of carbon used is usually 2 to 20% by weight based on the weight of the porous composite particles. When the amount of the carbon used is less than 2% by weight, the obtained porous composite particles may be insufficient in adsorption ability and strength. When the amount of the carbon used is more than 20% by weight, the amount of the filling material may become comparatively small, resulting in insufficient catalytic of the obtained porous composite particles. In the consideration of both the catalytic and strength, the amount of the carbon used is preferably 5 to 20% by weight, more preferably 8 to 20% by weight based on the weight of the porous composite particles.
The porous composite particles of the present invention have an average pore diameter of usually not more than 5 nm when measured with respect to pores existing in both the surface and the inside portions of the composite particles. When the average pore diameter is more than 5 nm, it may become difficult to selectively adsorb aimed substances, thereby failing to exhibit an excellent catalytic activity. The lower limit of the average pore diameter is 1 nm. In the consideration of the size of the substances to be adsorbed thereinto, the average pore diameter of the porous composite particles is preferably 1 to 4 nm, more preferably 1 to 3 nm.
The porous composite particles of the present invention have an average particle size of usually 1 to 1,000 xcexcm, preferably 1 to 300 xcexcm, more preferably 1 to 150 xcexcm. When the average particle size is less than 1 xcexcm. the porous composite particles may tend to suffer from secondary aggregation, so that the amount of the composite particles filled into a catalyst-packed tower may become insufficient. When the average particle size is more than 1,000 xcexcm, the obtained porous composite particles may be deteriorated in mechanical strength. Such large composite particles having a deteriorated mechanical strength may tend to be sometimes damaged or broken when filled into a catalyst-packed tower.
The porous composite particles of the present invention have a BET specific surface area of usually 45 to 200 m2/g. Here, the BET specific surface area represents a sum of a whole outer surface area and a whole pore surface area of the particles. When the BET specific surface area of the composite particles is less than 45 m2/g, the whole pore surface area thereof may become too small so that the pore volume thereof also may become insufficient, resulting in deterioration in adsorption property. When the BET specific surface area is more than 200 m2/g, it may be difficult to selectively adsorb the aimed substances. In order to obtain an effective catalytic activity, the BET specific surface area of the porous composite particles is preferably 47 to 180 m2/g, more preferably 50 to 150 m2/g.
The porous composite particles of the present invention have a micropore surface area of usually 45 to 198 m2/g. Here, the micropore surface area represents a total of pore surface areas except for an outer surface area of the particles. When the micropore surface area of the composite particles is less than 45 m2/g, the pore volume thereof may become insufficient, resulting in deterioration in adsorption property. When the micropore surface area is more than 198 m2/g, it may be difficult to selectively adsorb the aimed substances. In order to obtain an effective catalytic activity, the micropore surface area of the porous composite particles is preferably 47 to 180 m2/g, more preferably 49 to 150 m2/g.
The porous composite particles of the present invention have a pore volume of usually 0.02 to 0.1 cc/g. When the pore volume of the porous composite particles is less than 0.02 cc/g, the adsorption property thereof may tend to be deteriorated. When the pore volume of the porous composite particles is more than 0.1 cc/g, it may be difficult to selectively adsorb the aimed substances. In the consideration of obtaining an effective catalytic activity, the pore volume of the porous composite particles is preferably 0.03 to 0.08 cc/g.
The porous composite particles of the present invention may have an elliptical or spherical shape and, therefore, can exhibit a high packing percentage and a high flowability. This facilitates the filling of the composite particles into a catalyst-packed tower. In the consideration of the filling property of the composite particles into the catalyst-packed tower, the use of spherical particles having a sphericity of usually 1.0 to 1.25 is preferred.
The porous composite particles of the present invention may have a methylene blue adsorption of usually not less than 70%, preferably not less than 75%. The upper limit of the methylene blue adsorption is 100%.
The porous composite particles of the present invention have a monochlorobenzene dechlorination percentage of usually not less than 70%, preferably not less than 75%, more preferably not less than 80%. The upper limit of the monochlorobenzene dechlorination percentage is 100%.
Next, the process for producing the porous composite particles according to the present invention is described.
The porous composite particles of the present invention can be produced by polymerizing phenol with formalin in an aqueous medium in the presence of a filling material to produce granulated composite particles comprising, as a binder, a phenol resin containing a water-soluble phenol resin-forming component; drying the obtained granulated composite particles to adjust the content of the water-soluble phenol resin-forming component to a specific range; and heat-treating the dried composite particles in an inert atmosphere to carbonize the phenol resin.
In the process of the present invention, as the filling material, in addition to the above-described inorganic compound particles, there may also be used metal oxide hydroxide particles capable of being converted into the inorganic compound particles when heated at a temperature of not less than 500xc2x0 C., such as iron oxide hydroxide particles, e.g., goethite particles, akaganeite particles, lepidocrocite particles or the like.
The filling material may have any suitable shape such as a cubic shape, a polyhedral shape, a spherical shape, an acicular shape, a plate-like shape or the like. The filling material may have an average particle size smaller than that of the composite particles, usually 0.01 to 5.0 xcexcm, preferably 0.1 to 2.0 xcexcm. These filling materials may be used alone or in the form of a mixture of any two or more thereof.
The filling material used in the present invention may be preliminarily subjected to lipophilic treatment, if required. When such a lipophilic-treated filling material is used, it is possible to increase the amount of the filling material contained in the granulated composite particles, so that the obtained granulated composite particles tend to have a spherical shape.
The lipophilic treatment may be conducted by the method of treating the filling material with a coupling agent such as silane-based coupling agents and titanate-based coupling agents; or the method of dispersing the filling material in an aqueous medium containing a surfactant so as to adsorb the surfactant onto the surfaces of the particles.
As the silane-based coupling agents, there may be used those having hydrophobic groups, epoxy groups or amino groups. Examples of the silane-based coupling agents having hydrophobic groups may include vinyl trichlorosilane, vinyl triethoxysilane, vinyl tris(xcex2-methoxy)silane or the like. As the titanate-based coupling agents, there may be used isopropyl triisostearoyl titanate, isopropyl tridodecylbenzenesulfonyl titanate, isopropyl tris(dioctylpyrophosphate)titanate or the like.
Examples of the silane-based coupling agents having epoxy groups may include xcex3-glycidoxypropylmethyl dimethoxysilane, xcex3-glycidoxypropyl trimethoxysilane, xcex2-(3,4-epoxycyclohexyl)trimethoxysilane or the like.
Examples of the silane-based coupling agents having amino groups may include xcex3-aminopropyl trimethoxysilane, N-xcex2-(aminoethyl)-xcex3-aminopropyl trimethoxysilane, N-xcex2-(aminoethyl)-xcex3-aminopropylmethyl dimethoxysilane, N-phenyl-xcex3-aminopropyl trimethoxysilane or the like.
As the surfactants, there may be used commercially available surfactants, preferably those having functional groups capable of bonding with the filling material or hydroxyl groups existing on the surface thereof. As to the ionicity of the surfactants, the use of cationic or anionic surfactants is preferred.
The lipophilic treatment for the filling material may be conducted by any of the above-described methods. However, in the consideration of adhesion property to the phenol resin, the treatment is preferably conducted using silane-based coupling agents having amino and/or epoxy groups.
The amount of the lipophilic agent used is usually not more than 5.0% by weight, preferably 0.1 to 5.0% by weight based on the weight of the filling material. When the amount of the lipophilic agent is less than 0.1% by weight, it may be difficult to obtain the effect of increasing the amount of the filling material contained in the granulated composite particles. When the amount of the lipophilic agent is more than 5.0% by weight, an excessive amount of the lipophilic agent remains on the surfaces of the granulated composite particles, so that the composite particles may tend to be adhered to each other and agglomerated together via the lipophilic agent. As a result, it may be difficult to control the particle size of the granulated composite particles.
The granulated composite particles may be produced by polymerizing phenols with aldehydes in an aqueous medium in the presence of the filling material using a basic catalyst as an initiator.
As the phenols, in addition to phenol, there may be used compounds having phenolic hydroxyl groups, e.g., alkyl phenols such as m-cresol, p-tert-butyl phenol, o-propyl phenol, resorcinol and bisphenol A; halogenated phenols formed by substituting a part or whole of hydrogen atoms bonded to benzene ring or alkyl groups of the phenols, with chlorine or bromine; or the like. In the case where the compounds other than phenol is used, particles may be difficult to produce, or even if particles are produced, the obtained particles may tend to have an amorphous shape. Therefore, in the consideration of the particle shape, the use of phenol is preferred.
As the aldehydes, there may be used formaldehyde in the form of either formalin or paraformaldehyde, furfural or the like. Among these aldehydes, formaldehyde is preferred.
The molar ratio of the aldehydes to the phenols is preferably 1:1 to 4:1, more preferably 1.2:1 to 3:1. When the molar ratio of the aldehydes to the phenols is less than 1:1, it may tend to be difficult to produce the granulated composite particles, or even if the particles are produced, there may be such a tendency that the obtained granulated composite particles have a low strength since the curing of resins is difficult to proceed. When the molar ratio of the aldehydes to the phenols is more than 4:1, an increased amount of unreacted aldehydes may tend to still remain in the aqueous medium after the reaction.
As the basic catalysts, there may be used those used for the production of ordinary resol resins. Examples of the basic catalysts may include aqueous ammonia or alkyl amines such as hexamethylene tetramine, dimethyl amine, diethyl triamine, polyethylene imine or the like. The molar ratio of the basic catalyst to the phenols is usually 0.02:1 to 0.7:1.
In the present invention, the granulated composite particles may be produced in the form of a water dispersion by conducting the reaction between the phenols and the aldehydes at a temperature of usually 70 to 90xc2x0 C. simultaneously with the curing reaction, and then cooling the obtained particles to a temperature of not more than 40xc2x0 C.
Then, the water dispersion (reaction mixture) containing the granulated composite particles is subjected to solid-liquid separation by ordinary methods such as filtration and centrifugal separation. Thereafter, the thus separated particles are dried to obtain dried granulated composite particles.
Upon drying the granulated composite particles, it is essential to control the drying conditions such that an appropriate amount of a water-soluble phenol resin-forming component such as phenols, aldehydes and oligomers having some degree of polymerization, still remain in the dried particles.
In order to control the amount of the water-soluble phenol resin-forming component contained in the dried composite particles to a specific range, the water-washing of the particles obtained by the solid-liquid separation should be avoided. That is, in order to control the amount of the water-soluble phenol resin-forming component contained in the dried composite particles to a specific range, the particles is dried under moderate conditions so as to remove only water therefrom, for example, at a temperature as low as 50 to 100xc2x0 C. under atmospheric pressure. The amount of the water-soluble phenol resin-forming component contained in the dried composite particles is usually 0.6 to 5 mg/liter, preferably 0.6 to 3 mg/liter, more preferably 0.7 to 2 mg/liter, when measured according to the test method for utensils or toys 3,1,5,3 defined in Japanese Food Sanitation Law.
When the amount of the water-soluble phenol resin-forming component is less than 0.6 mg/liter, the obtained composite particles may not have a sufficient amount of pores in the surface and inside portions thereof. When the amount of the water-soluble phenol resin-forming component is more than 5 mg/liter, the particles may tend to be agglomerated together upon the subsequent heat-treatment for carbonizing the phenol resin.
In the present invention, the polymerization reaction may be conducted in the presence of a suspension stabilizer, if required. The presence of the suspension stabilizer facilitates the production of the granulated composite particles.
Examples of the suspension stabilizers may include hydrophilic organic compounds such as carboxymethyl cellulose and polyvinyl alcohol; fluorine compounds such as calcium fluoride; water-insoluble inorganic salts such as calcium sulfate; or the like.
In the process of the present invention, the dried composite particles may be then heat-treated to decompose and carbonize the phenol resin at a temperature of usually not less than 400xc2x0 C., preferably 500 to 800xc2x0 C. in an inert atmosphere.
When the heat-treating temperature is less than 400xc2x0 C., the carbonization of the phenol resin may require a long period of time. For example, in the case where magnetite is used as the filling material, the magnetite particles tend to be partially or entirely reduced into readily-oxidizable iron or wustite by the resins when the heat-treatment is conducted at a too high temperature. For this reason, in case of the magnetite particles, the heat-treatment is preferably conducted at a temperature of not more than 800xc2x0 C.
As the heat-treating furnace for the above heat-treatment, there may be used any suitable furnaces such as fixed-type ones or rotary-type ones. Among these heat-treating furnaces, the use of rotary-type heat-treating furnaces is preferred.
The inert atmosphere may be produced by flowing an inert gas such as helium, argon and nitrogen through the heat-treating furnace. Among these inert gases, the nitrogen gas is sufficient in view of costs. Incidentally, when the heat-treatment is conducted in an oxidation atmosphere, e.g., in air, the carbon is reacted with oxygen to produce carbon dioxide so that the actual carbon ratio (amount of residual carbon contained in the composite particles; hereinafter referred to merely as xe2x80x9cactual carbon ratioxe2x80x9d.) are lowered, thereby failing to accomplish the aims of the present invention.
The flow rate of the inert gas varies depending upon scale or size of the heat-treating furnace used. Even when using readily-oxidizable filling materials such as metal iron or magnetite, the oxidation problem can be sufficiently avoided by flowing the inert gas at a flow rate of usually not less than one liter/min., preferably 2 to 8 liters/min.
The heat-treating time varies depending upon the heat-treating temperature used, and is usually 1 to 3 hours.
The important point of the present invention lies in the fact that by polymerizing phenols with aldehydes in an aqueous medium in the presence of the filling material using a basic catalyst as an initiator; drying the resultant particles to obtain dried composite particles containing a phenol resin as a binder resin, to adjust the amount of a water-soluble phenol resin-forming component contained in the dried composite particles to a specific range; and heat-treating the obtained dried composite particles at a temperature of not less than 400xc2x0 C. in an inert atmosphere to carbonize the phenol resin, there can be obtained porous composite particles having an average pore diameter of not more than 5 nm, when measured with respect to pores existing in both the surface and inside portions thereof.
The reason why the porous composite particles of the present invention have fine pores, is considered as follow. That is, the phenol resin contained in the dried composite particles has a high actual carbon ratio, and the water-soluble phenol resin-forming component contained in the dried composite particles in an adequate amount can be removed by the subsequent heat-treatment.
The reason why the porous composite particles of the present invention have pores having uniform pore diameters, is considered by the present inventors as follows. That is, in the granulated composite particles obtained from the aqueous solution by the polymerization reaction, the filling material and the phenol resin are uniformly dispersed therein, so that the water-soluble phenol resin-forming component is also uniformly contained in the composite particles.
The reason why the porous composite particles of the present invention exhibit a high strength, is considered as follows. That is, the filling materials are uniformly and firmly bonded together via the phenol resin, thereby producing granulated composite particles having a closely packed structure. Since such granulated composite particles are used as raw particles to be heat-treated, the obtained porous composite particles can also have a closely packed structure, resulting in a high strength thereof.
The porous composite particles according to the present invention contain a large amount of the filling material having various catalyst property and, therefore, can exhibit a high catalytic. In addition, the porous composite particles according to the present invention contain fine pores in the surfaces and inside portions thereof, thereby exhibiting a high adsorption ability. Therefore, the porous composite particles of the present invention are suitably used as porous composite particles for catalysts.
In the case where the porous composite particles of the present invention are used as various catalysts, since a large amount of the filling material having a good catalytic is contained therein and since pores having uniform diameters are formed therein, it is expected that the porous composite particles can exhibit an excellent catalytic activity, for example, when used as adsorbents or catalysts for effectively removing chlorine contained in liquefied decomposition oils obtained by thermal decomposition of waste plastics, or chlorine contained in water.